1. Field of the Invention
The present invention relates generally to apparatus for controlling flow rates of liquids and, more particularly, to apparatus for maintaining accurate, precise and stable liquid flow rates in medical liquid delivery systems, such as intravenous (IV) fluid delivery systems.
2. Background Discussion
Many applications exist in which the precisely-controlled flow of small quantities of liquid materials is essential. One particularly critical example of such applications is the required precise delivery of controlled amounts of intravenous (IV) liquids to patients in hospitals, clinics, or in the field.
Gravity-driven IV liquid delivery systems or devices known to the present inventor utilize variable orifice flow control valves to control the flow rate of fluids to a patient undergoing treatment. Such known valves require a constant pressure drop, .DELTA.P, across the orifice to maintain a stable, accurate and precise flow rate of fluid into patients to which the systems or devices are connected.
Some of these known flow control systems or devices utilize one or more fixed orifices and adjust the pressure drop, .DELTA.P, across the orifice(s) to set the fluid flow rate. This pressure drop (the absolute sum of the positive and negative pressure) across the orifice(s) calibrated for a liquid of known viscosity and density determines the instantaneous fluid flow rate through the orifice(s).
Many known variable and fixed orifice fluid flow control systems attempt to circumvent or accommodate the normal decrease in positive fluid head pressure as the supply liquid level drops as, for example, liquid is drained from an IV bottle, and also variations in negative or suction pressure in the patient delivery line, as may, for example, be caused by patient movement or changes in the patient's venous pressure.
Variable orifice flow control valves of some known IV flow control devices are marked with flow rates that appear to assume an average pressure drop, .DELTA.P, across the orifice, with no control of supply head pressure or patient line suction. For example, the fluid flow control valves disclosed in U.S. Pat. Nos. 4,789,000; 4,802,506 and 4,807,660 are considered representative of this type device. Instructions provided with commercial versions of the just-mentioned type of IV flow control devices caution users initially to count the number of liquid drops falling through an associated drip chamber in a prescribed period of time to establish an accurate flow rate, and thereafter to adjust the valve frequently to maintain a relatively constant liquid delivery rate as the supply liquid head and/or the patient line pressures changes.
Other known IV flow rate control systems, such as those disclosed in U.S. Pat. Nos. 3,929,157; 4,340,050 and 4,588,396, disclose or suggest controlling fluid head pressure by transferring liquid from a primary liquid supply source into a secondary vessel in which the level is held constant and independent of the decreasing head pressure of the primary liquid source as it empties.
Other examples of known IV devices are disclosed in U.S. Pat. No. 3,929,157. These particular patents disclose IV devices in which a tube connects a rigid supply source to a secondary chamber for head pressure control. Liquid flows under gravity from the supply source into the secondary chamber until the bottom of the tube is covered. At that point air can no longer pass up the tube to displace the in-flowing liquid and flow stops. The region above the liquid in the secondary chamber is connected to the atmosphere (that is, the region is at atmospheric pressure), so there is no coupling through the air between the supply head pressure and the head pressure in the chamber.
The head pressure on a fixed outlet orifice located in the bottom of the secondary chamber determines the rate of flow, which may be adjusted by sliding the secondary chamber up or down on the tube from the supply source, thereby adjusting the head pressure of the liquid in the secondary chamber. A liquid collection chamber below the orifice collects the liquid and a flexible tube conducts the liquid from the collection chamber to a patient. This collection chamber is also vented to atmosphere so that changes in the liquid height in the patient line or changes in venous back pressure are uncoupled from the orifice and will have no effect on the flow rate through the orifice.
U.S. Pat. No. 4,340,050 discloses the use of a collapsible bag for the supply source. The bag discharges liquid into a liquid-receiving chamber which is vented to the atmosphere. A float-type valve is pivotally mounted in the liquid-receiving chamber for maintaining a constant liquid level. As disclosed, a second chamber, having fixed orifices at various heights and which is fluidly connected to the liquid-receiving chamber, can be moved up and down relative to the liquid-receiving chamber to vary the head pressure on the orifices. Liquid passing through the orifices collects in the bottom of the second chamber and is conducted to a patient through a flexible IV tube. This second chamber is vented to the atmosphere above the liquid on both sides of the orifices, thereby uncoupling the orifices from any pressure changes in the IV line connected to the patient.
In another example of the known IV flow control art, U.S. Pat. No. 4,588,396 discloses the use of a tube which connects a rigid supply source to a liquid-receiving chamber in which a constant liquid level is maintained in the manner disclosed in above-mentioned U.S. Pat. No. 3,929,157. The air above the liquid in the receiving chamber, which is the source of displacement air in the supply vessel, is vented to atmosphere through a metering valve which is used for flow rate control, instead of an orifice being used in the IV liquid path. Liquid is disclosed as flowing out of this collection chamber through a sealed drip chamber which is connected to a patient delivery line. It appears, however, that changes in the height of the liquid in the patient line will couple through the air in the sealed drip chamber to the liquid in the collection chamber and affect the flow rate through the system.
Other known IV flow rate control systems, such as are disclosed in U.S. Pat. Nos. 4,142,523; 4,186,740; 4,515,588 and 4,863,437, use a diaphragm or collapsible chamber which adjusts a flow control orifice or passage to minimize flow rate changes caused by supply head pressure and patient delivery line pressure variations. As far as the present inventor is aware, however, none of such disclosed IV devices isolate the flow control element from both the supply head and patient line pressure variations.
By way of a still further example, U.S. Pat. No. 4,613,325 discloses an IV flow rate control system that amplifies and uses a velocity dependent pressure drop across a restriction in the flow path to modulate the size of an upstream variable orifice flow control. There does not, however, appear to be disclosed any means for isolating the flow restriction from supply head or patient line pressure variations.
Further examples of known IV flow control devices are disclosed in U.S. Pat. Nos. 5,014,750 and 5,033,714. These patents disclose a pressurized constant pressure liquid supply that is fed through an adjustable flow restricter into a patient delivery line, the entire IV system being worn by the patient. However, since the system is tied to the patient, little variation would be expected in patient line pressure except that caused by venous blood pressure.
Many of the known IV flow rate control devices that attempt to compensate for changes in supply head pressure and patient line pressure are expensive and complex, and the range of pressure compensation before a nurse must reset the flow rate is considered by the present inventor to be limited, particularly, since in a "worst case" situation a patient's line suction on the flow control orifice can, depending, for example, on the patient's position (e.g., standing, sitting or lying) increase by 30 to 40 cm of water, possibly doubling the pressure drop across the orifice.
An important need, therefore, still exists in the medical field for an improved, simple, reliable and relatively low-cost, gravity-driven IV system that delivers an accurate, precise and stable flow rate of medicinal liquid to patients, in bed or ambulatory, in hospital, home, field or transport settings. It is, therefore, a principle objective of the present invention to provide such an improved IV system.